No income tax levied by the federal government, the several States, or any subdivision of either shall ever apply varying rates to those in its jurisdiction.
If this means levels of government gets to tax up to 10% each, then the result may well be more layers. Conversely if this means that 10% is the combined limit, you need to have some kind of collaboration procedure between the government levels. For example, say local government tax is at 1%, state is at 2% and the fed is at 7%. Then the fed wants to raise it to 9%...what happens? Are they stuck unless they convince each state or each local government to make an adjustment? Or perhaps the states must then lower theirs to 1% and then the local governments must eliminate theirs altogether?
opps...meant the 10% one
This is a possibility; but it is not necessarily the case — consider states like TX which do not have an income tax, conversely some Cities (e.g. NYC) have income taxes [IIRC].
I haven't thought of a better way to address the problem of graduated income taxes as well as the various subdivisions.
Perhaps altering Sec V to read:
No income tax levied by the federal government and its subdivisions shall exceed 10%, neither shall the income tax of a State and its subdivisions exceed 10%.